Thus, if you were a party who wanted to promise cutting taxes to the population you could, but you'd have very little impact on the taxation's bottom line. I actually don't know what the Barnett formula says about money the Scottish government doesn't spend from Westminster (wondering if you could in theory as a party in Holyrood decide just not to spend the money you were given, as a forcing tactic).
Regardless, all parties (in both London and Edinburgh) and the Scottish population are in agreement, the situation of controlling spending but not tax raising is an unfit state for a government to be in.
[edit - make my 'all parties' comment clearer]